Manufacturing of integrated circuits is generally a procedure of forming thin films and layers of various materials on wafers of base semiconductor material, and selectively removing areas of such films to provide structures and circuitry. Doped silicon is a typical base wafer material. CVD is a well known process for depositing such thin films and layers. For example, polysilicon may be deposited from silane gas, SiH.sub.4. It is known, too, to deposit tungsten silicide from a mixture of gases including silane and a tungsten-bearing gas such as tungsten hexafluoride. Pure tungsten is also deposited on silicon wafers in the manufacture of integrated circuits, sometimes selectively and sometimes across the entire surface in a process known as "blanket" tungsten.
In a typical CVD process such as blanket tungsten, wafers are placed on supports within a sealable chamber, the chamber is sealed and evacuated, the wafers are heated, typically by heating the wafer support, and a gas mixture is introduced into the chamber. For example, in the blanket tungsten process, tungsten hexafluoride and hydrogen are introduced as reactive gases, and typically argon is introduced as a non-reactive carrier gas. The tungsten hexafluoride is the source of deposited tungsten. Typically the gases are flowed continuously during process. The temperature of a substrate (wafer) to be coated is one of the variables that drives the chemical reaction to cause tungsten to be deposited on the substrate surface. It is important to control the temperature, the concentration of various gases in the mixture introduced, and such characteristics as the uniformity of flow of gas over the surface being coated, among other variables.
In the practice of CVD deposition of tungsten on wafers in the manufacture of integrated circuits, it is common for the silicon substrate to have portions of an integrated circuit already formed thereon, and in a typical application, an insulating layer of silicon oxide has been formed over transistor devices formed on the wafer and patterned to provide contact openings or vias giving access to underlying structures. Tungsten deposition in this case has two related purposes: one is to fill the vias to provide electrical contact to lower levels, and the other is to provide a thin metal film to be subsequently patterned and etched to provide electrically connecting traces between devices on the wafer, providing thereby the integrated circuit.
In recent years a number of studies have been carried out and published on high throughput blanket tungsten CVD processes for contact/via fill applications, including "plugs", and also on interconnect applications. An example is a paper "Workshop on Tungsten and Other Advanced Metals for VLSI/ULSI Applications V", by R. V. Joshi, E. Mether, M. Chow, M. Ishaq, S. Kang, P. Geraghty, and J. McInerney; edited by S. S. Wong and S, Furukawa, published in Materials Research Society, pp. 157 (1990), incorporated herein by reference. For the purpose of this section, the two types of applications of blanket tungsten process will be termed "plug" and "interconnect".
Experience to the present time, including the study above and other studies, indicate that the two basis applications for blanket tungsten have considerably different requirements in terms of film characteristics. For example, low stress films are known to be quite important for interconnect applications, but stress is not as critical for plug applications. Similarly, high reflectivity is desirable for interconnect processes, but not as critical for plug processes. Low resistivity is more important for interconnect processes than for plug processes. Finally good step coverage is important in both types of processes.
Given the relative importance of different film characteristics as listed above, it is known also that film characteristics vary in some generally known ways depending on process parameters such as wafer temperature, total process pressure, and Nitrogen flow rate. For example, studies and experience show that low stress films are promoted by relatively higher wafer temperature during deposition and by low process pressure, generally under 1 Torr. As a baseline, a stress level of 7.times.10.sup.9 dynes/cm.sup.2 is a desirable goal, and in the experience and knowledge of the present inventors, requires deposition temperature well above 480.degree. C. There are processes, however, wherein the devices and materials already formed on a wafer to be coated by tungsten, may be adversely susceptible to high temperature. In most cases this threshold is about 440 degrees C.
Reflectivity is promoted generally by a low flow rate of WF.sub.6, low deposition temperature (400.degree. C.), high process pressure (100 Torr), and a high flow rate of nitrogen in the process gas. Reflectivity of 40% and greater is desirable in subsequent photolithography processes, measured relative to silicon at 436 nanometer wavelength for 1 micron film thickness.
Low resistivity (less than 9 micro-ohm-cm) is enhanced by high deposition temperature and little or no nitrogen in the process.
Maximum step coverage effect is promoted by a high flow rate of WF.sub.6, low wafer temperature, and medium to high process pressure.
Because of the different requirements for film characteristics and the dependence on process parameters, generally two types of processes have been developed. One type for plug processes, with relatively high film stress, superior step coverage, using moderate temperature, and the other for interconnect applications, with low stress film having high reflectivity and providing moderate step coverage, performed at higher temperature.
Experience has shown that even the optimization of process for each application still leaves problems. For example, in interconnect processes, the effort to decrease film stress leads to reduced step coverage and difficulty with small contact areas such as 0.5 microns and below. Moreover, there has been evidence that high wafer temperature may cause junction leakage, as specified by T. Kaelyama, Y. Imamura, and K. Hamamoto in "The Proceedings of the 8th Symposium on Ion Beam Technology", 1989, pp141, at Hossei University in Tokyo, Japan. There are similarly other difficulties not covered in this brief description of the background.
What is clearly needed is a blanket tungsten process at low temperature, below 440.degree. C., and preferably as low as 400.degree. C., which is suitable for both interconnect and via fill, or "plug" applications. In such a "universal" process, it would be desirable to have an efficient use of WF.sub.6 reactant, the expensive source of tungsten. It would be desirable, too, if the process could be accomplished with adequate reflectivity but without the use of nitrogen gas, as nitrogen is known to promote poor film adhesion in some instances, particularly in reactor walls and parts which increases particulate problems, and also to increase resistivity of film intended for interconnects.